Odd behavior of preamp

[Dwiel]

Hello, I'm working on a preamp for instruments such as guitar, bass, etc. The preamp is actually not to be output to any sound but actually to some circuits which detect volume, frequency, attack, etc and produce some kind of light output. The main problem that I am currently running into involves some kind of interfierence getting into the signal. It only happens when I touch the strings of the guitar with my hand. The ammount of interfierence that I get depends on where the volume knob on the guitar is turned to and oddly enough is worst somewhere in the middle of the volume. When volume is very low, there isn't a problem and when the volume is at maximum, there isn't a problem.

I am measuring this interfierence at the output of my opamp preamp. At the worst position, I get 1.8VAC out of the preamp without even touching the guitar, 2.3VAC when I touch the strings, and a little more than 1.8VAC when I strum it. When the volume is placed at maximum on the guitar, I get 0.1VAC at rest, 0.4VAC when I touch the strings and up to 1.5VAC when I strum loudly. I have attached the schematic of the circuit that I am currently using.

One thing I thought of is that I am using general purpose CMOS opamps (mcp6004) could this be the cause of the problem?

Thanks

[Tim__x]

R5 needs to go to the other side of C3. Also, the gain is a little high.

Moving R5 might solve your problem, try it out and come back if it still doesn't work.

[Dwiel]

Ah yeah, thanks for pointing that out. That actually didn't really change much with this problem... I do have the gain set high to make it easy to adjust the comparator to an exceptable voltage reference in order to create the final trigger.

I tired replacing the 4.7k with a 50k pot in order to allow adjusting the gain. I actually determined after a little more testing that even if I just connect my multimeter to the guitar output directly I get the following voltages (when volume knob is placed in worst case middle position):

0.1vac untouched
0.3vac when I touch it
< 0.2vac when I strum it.

When I put this guitar through my amp, I get no such audible signals simply by touching it.

I'm not sure how this signal can be turning up on my voltmeter and making it through my opamp and comparator but somehow the guitar amp is rejecting it.

Thanks!

[Enzo]

I know it is not the intended purpose, but for troubleshooting purposes, connect the output of your preamp to a guitar amp so you can HEAR the noise. I suspect it is hum, and grounding and shielding is likely the issue.

[poobah]

Hmmmm...

[Dwiel]

You are correct in assuming that is a hum that is getting throgh. I measured the output frequency and it turned out to be 60 hz. I also connected my amp to the circuit and heard it very clearly. One thing that baffles me is that I tried simply going from my computer's line out directly to my prototype board and from there to the amp. Didn't even connect the ground to my power supply ground or the signal to the input of my preamp. And I ended up with the same hum.

This means that somehow the hum is getting picked up from my prototype board? How can I avoid this? I have no idea how I would prototype this circuit up if I couldn't use my prototype board.

Thanks

Zach

[Dwiel]

One thing that I have noticed is that when I bypassed everything and went from computer line out -> protoboard -> guitar amp and I could here lots of hum. However when I go from line-out -> my preamp -> VU meter I get acceptable levels without any indication of hum. Atleast it isn't loud enough to be interfiering with my VU meter. The problem only seems to be when I use my guitar as a source. Something to do with the input levels or the impedence? I'm not really sure

Zach

[poobah]

You can experiment very easily by wrapping your circuit in a clean dry towel followed by a layer of aluminum foil connected to the input ground... costs almost nothing... gives quick answers.